1. Technical Field
The invention relates to a method for producing a magnesium-rare earth intermediate alloy, which belongs to the technical field of molten salt electrolytic metallurgical technology.
2. Background of the Art
Magnesium-rare earth intermediate alloy is a basic material for producing an advanced and new type of corrosion-resistant and high temperature-resistant magnesium alloy. There are mainly the following three methods for producing a magnesium-rare earth intermediate alloy. The first one is mutual infiltration method, and the second one is magnesium thermal reduction method, but these two methods have the following defects: it is difficult for the mutual infiltration method to avoid segregation of the alloy components, since magnesium is greatly different from most of rare earth metals in specific gravity and melting point, so that rare earth metals cannot be dispersed well into magnesium; while the magnesium thermal reduction method is a batch method, which production cost is high. The third one is molten salt electrolytic method, which comprised both of liquid state cathode method and co-precipitation electrolytic method.
Liquid state cathode method is a method for producing a magnesium-rare earth intermediate alloy by electrolysis using magnesium or magnesium-rare earth intermediate alloy having a low rare earth content as liquid state cathode. Electrolysis is carried out by using the alloy constituents as the cathode, wherein rare earth ions in the electrolyte is migrated and dispersed toward the cathode under the action of DC electric field, and an electrochemical reaction is carried out on the cathode. The rare earth segregated on the cathode is alloyed with the cathode magnesium to produce the magnesium-rare earth intermediate alloy having a low melting point.
Ping LI, Dingxiang TANG, et al. proposed a method for producing an yttrium-rich rare earth-magnesium alloy having 20 to 30% of yttrium-rich rare earth, by using an alloy having 10 wt % of yttrium-rich rare earth as a sinking cathode, in a electrolyte of 15 to 20% of RE(Y)Cl3—KCl—NaCl, under an electrolysis condition of a cathode current density being 1 to 1.5 A/cm2, with a current efficiency of greater than 70%[1].
Chunxu REN and Kangning ZHANG proposed a method for electrolyzing neodymium magnesium alloy using liquid state metal magnesium as a cathode. Magnesium is floated on the surface of the electrolyte because of its lower density, and becomes the upper liquid cathode. NdCl3—KCl—NaCl is used as the electrolyte, the content of NdCl3 is 20%. The electrolysis temperature is 820±20° C., and the cathode current density is 1.5 A/cm2. At the early stage of electrolysis, the magnesium cathode is floated on the tope of the electrolyte. During the electrolysis, as neodymium is segregated continuously and formed into a magnesium neodymium alloy with the liquid state cathode, the density of the cathode alloy increases with the content of neodymium. When its density is greater than that of the electrolyte, the alloy cathode sinks into a bottom receiver. At this moment, the cathode conductive molybdenum bar should also fall down along with the alloy cathode so as to keep contact with the alloy. During the electrolysis, the alloy is stirred continuously, which can accelerate the dispersion of neodymium into the inside of the alloy, enhance the alloying process, eliminate the concentration gradient of the alloy, and increase the current efficiency and the direct yield of neodymium, wherein the neodymium content in the magnesium alloy can be up to about 30%. The current efficiency of the process is 65-70%, and the neodymium direct yield is up to 80-90%[2].
The co-precipitation electrolytic method is a method for producing a magnesium-rare earth intermediate alloy by using the co-precipitation of the rare earth ions and magnesium ion in the electrolyte onto the cathode.
Yttrium magnesium and yttrium magnesium-rich alloys were produced in Zhongshan University, by electrolytically coprecipitating in a melt containing yttrium chloride and yttrium-rich chloride, in a small type of graphite electrolysis cell. In the case where the temperature is 850 to 860° C., the cathode current density is 20 to 32 A/cm2, and 25 to 35% of YCl3 and 4 to 6% of MgCl2 is contained in the melt, an yttrium magnesium alloy containing about 60% of yttrium can be obtained through electrolysis, wherein the average current efficiency is 70%, and the yttrium direct yield is 75%, with a maximum being able to be up to 83%.
A yttrium magnesium alloy having a yttrium amount of more than 60% is obtained in Hunan Rare Earth Material Institute, by using YCl3—MgCl2—KCl as the electrolyte, and electrolyzing at a temperature of 900° C., in a graphite electrolysis cell having a diameter of 150 mm, wherein the current efficiency is 50%, and the rare earth direct yield is greater than 70%[4].
In Baotou Rare Earth Research Institute, a Mg—Ce intermediate alloy containing 40 to 60% of Ce is produced under an order of 800 A, by using CeCl3 crystalline material and anhydrous MgCl2, with CeCl3—MgCl2—KCl in a ratio of CeCl3/MgCl2/KCl=25-35/3-5/60-70 (wt %) being the electrolyte, at a temperature of 900 to 920° C. and under a cathode current density of 10 to 15 A/cm2, wherein the current efficiency is up to 75%, the direct yield of the rare earth is 95%, and the direct yield of magnesium is 98%[5].
However, the raw materials used is anhydrous rare earth metal chloride in the liquid state cathode method, and anhydrous magnesium chloride in the electrolytic co-precipitation method, which require a complicated dehydrating process, especially the process for removing the final 2 crystalline waters from magnesium chloride is extremely complicated, causing many problems such as large energy consumption, large material consumption and the equipment corrosion by HCl.